I have a 2011 MacBook Pro that came with Snow Leopard. Couple of months back I upgraded to Lion. I now wish to put Snow Leopard back on a 30GB partition but no matter what I do it simply won't install. If I try and reboot holding down C it just goes back into Lion. I had to set the boot disk as the DVD and then it worked... right up until it tried to run off the disc and had a black-screen-of-death kernel panel (which incidentally had the words 'unsupported CPU' in the error message). I've tried both the disc that came with the machine and a retail version, same issue. I don't see why there would be a problem; it came with Snow Leopard to begin with!